It is common practice to periodically insert a fixed synchronization word in a data stream (using time-division multiplexing) so as to assist the various receivers in a system in achieving the timing and frequency necessary to successfully demodulate the received signal. It is desirable to use this fixed code word (ideally having pseudo-random properties leading to an aperiodic autocorrelation function with low sidelobes) since it can speed acquisition as compared to what can be done by directly synchronizing to the data stream which may not have a good autocorrelation function. However, use of this dedicated synchronization word does use a portion of time and communication resources that would otherwise have been available for the data stream. Consequently, it is desirable to minimize the length and the duty factor of this periodic synchronization signal.
On the other hand, in many applications, it is important to acquire this synchronization signal quickly in the face of unknown time and large frequency uncertainty with a relatively simple receiver. A good example of this situation is in satellite-based cellular telephone service. Most such systems use low- or medium-earth orbit altitude satellites to reduce the transmit power required from the mobile units. The use of such orbits results in substantial frequency uncertainty on the order of several to many tens of kHz. Often, due to the weak link budgets, the data rates for these systems is very low (on the order of 4.8 kb/s) and the signals are heavily coded. As a result the received carrier-power-to-noise-power-spectral-density ratio, or C/No, is very low. Thus, it is necessary to correlate the received signal over a substantial length of time to achieve a sufficiently high energy-per-synchronization-word-to-noise-power-spectral-density-ratio to permit reliable acquisition of the synchronization word. If a single matched-filter detector is used for acquisition it will be necessary to have a very accurate frequency estimate for good performance. The accuracy of the frequency estimate is inversely proportional to the length of the synchronization word. With a single matched filter MF, this means that it would be necessary to try many different frequency bins. In many applications this approach results in unacceptably long acquisition process. FIG. 1 illustrates a block diagram of this sequential approach where a voltage-controlled oscillator (VCO) is used to select the frequency bin.
An alternative to a single matched filter is the use of multiple matched filters MF1, MF2 . . . MFN--each one tuned to one of the many frequency bins. FIG. 2 is a block diagram of the parallel approach. Unfortunately, each matched filter is reasonably complex and the requirement for multiple units, e.g., 20 more, results in excessive equipment and power consumption. For the mobile phone example, power consumption is of great concern since the mobile phones are battery operated.
Thus, significant problems exist with the conventional sequential and parallel approaches to the acquisition of the synchronization word. The invention presented herein solves these problems resulting in the minimum length synchronization word yielding the desired reliability of acquisition with a receiver of acceptable complexity and power consumption.
To this point, for tutorial purposes, the background material has selected the example of time division multiplexing for the insertion of the synchronization word in a data stream. There are other applications, e.g., ranging systems, in which there is no data to be transmitted, and, also, there are alternative methods (to time division multiplexing) for combining the data and the synchronization signal. Those skilled in the art will easily recognize that the innovative concepts for rapid acquisition presented herein are directly applicable to such problems.